Polyesters typified by polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate excel in mechanical properties and chemical properties and are used in a wide variety of fields including fibers for clothes and industrial materials, films or sheets for packaging materials or magnetic tapes, bottles, which are hollow molded articles, casings of electric or electronic appliances, and other types of molded articles or components.
Certain representative polyesters, namely, polyesters composed of aromatic dicarboxylic acid components and alkylene glycol components as major constituents, such as polyethylene terephthalate, are produced by first preparing bis(2-hydroxyethyl)terephthalate (BHET) and an oligomer containing the same by an esterification reaction between terephthalic acid and ethylene glycol or transesterification of dimethyl terephthalate and ethylene glycol, and then subjecting them to melt-polycondensation in vacuo at high temperatures in the presence of a polycondensation catalyst.
As such a polycondensation catalyst for producing polyester, antimony trioxide is heretofore widely used as disclosed in JP 46-3395 B. Antimony trioxide is a catalyst which is inexpensive and is of excellent catalytic activities, however, it has some problems. For example, antimony metal is formed while it is used in polycondensation thereby making the resulting polyester darkened, or the resulting polyester is contaminated with foreign substances. In addition, antimony trioxide is inherently poisonous. In recent years, therefore, development of catalysts free of antimony has been awaited.
For example, a catalyst composed of a germanium compound is known as a catalyst which has an excellent catalytic activity and which can provide polyester excellent in hue and thermal stability. This catalyst, however, is problematic in that it is very expensive and that the catalyst content in a reaction system changes with time and it becomes difficult to control the polymerization because the catalyst is easily distilled off from the reaction system during the polymerization.
On the other hand, as disclosed in JP 46-3395 B and JP 49-57092 A, it is already known that titanium compounds such as glycol titanate and titanium alkoxide also can be used as a polycondensation catalyst for producing polyester by transesterification of dimethyl terephthalate and ethylene glycol. For example, according to U.S. Pat. No. 5,596,069, polycondensation catalysts comprising tetraalkoxy titanate are known. They, however, have problems in that the resulting polyester is liable to be colored due to thermal degradation during the melt-molding thereof.
In recent years, many methods for producing high-quality polyester at high productivity using a titanium compound as a polycondensation catalyst have been proposed. For example, as disclosed in JP 2001-064377 A and JP 2001-114885 A, a solid titanium compound obtained by first preparing a hydroxide of titanium by hydrolysis of titanium halide or titanium alkoxide and then dehydrating and drying the hydroxide by heating it at a temperature of from 30 to 350° C. has been proposed as a polycondensation catalyst.
Some of the heretofore known polycondensation catalysts composed of titanic acid, including the above-mentioned titanium compounds, have high polymerization activities per unit weight of metal. However, in many cases, there is a tendency that such a catalyst exhibits a high polymerization activity, but the resulting polyester is liable to be colored due to thermal degradation during its melt-molding. There is also a tendency that the resulting polyester is poor in transparency.